For prospective motion correction, the motion of the examination object, such as, for example, a head movement of a patient, is detected during the acquisition of a volume and corrected in real time. Such prospective motion corrections are, for example, necessary during functional magnetic resonance (MR) examinations or during MR examinations for the compilation of parameter maps such as T1 maps in order to correct motions of the examination object that occur during the scan.
There are numerous possible ways for achieving this. One possibility is the use of image-based navigators. The use of navigators is, for example, suitable for long pulse sequences in which waiting times occur in which the additional navigator signal may be scanned. For example, low-resolution 3D EPI navigators may be used that may be recorded in approximately 500 ms. Each imaging sequence with dead times of longer than 500 ms may be provided with these navigators in order to perform real-time motion compensation. To achieve this, the EPI navigators are reconstructed and registered by image-based methods to a reference time, the navigator reference signal, for the determination of motion information. The calculated detected motion parameters of the current navigator are sent to the sequence to enable this to take account of the motion and, for example, adapt the position of the region of interest.
Very high requirements are placed on the detection of the motion parameters in order to be able to implement real-time motion correction. For this reason, rigid model assumptions with six degrees of freedom, three translation degrees of freedom, and three rotation degrees of freedom, which are justified in numerous applications, such as, for example, in the head, are made. Hence, for the motion detection, the navigator signals are compared to a navigator reference signal, or the navigator data set is registered to the navigator reference data set. Examples of techniques for motion detection are, for example, known from THESEN S. et al, “Prospective Acquisition Correction for Head Motion With Image-Based Tracking for Real-Time fMRI” in Magn. Reson. Med. 44 (2000) 457-465; such techniques may also be used in conjunction with the techniques described herein.
The same principle may be used without the use of an additional navigator signal by taking account of the MR signals for the generation of the MR image. This procedure may be used in the case of time-resolved volume data. In this case, an image data set of a subsequent time point is registered to a reference time point. The detected motion parameters of a time point t1 relative to a reference time t0 are compensated in the next time point t2. As a result, the motion compensation is delayed by at least one repetition time compared to the actual motion. This delay may sometimes be tolerated in the case of continuous, small motions, caused, for example, by breathing, but not in all fields of application.
During the recording of image-based navigators and the associated MR data, it may happen that, in the navigator volume, in which the navigator signals and navigator reference signals are recorded, regions occur that contain little to no image information so that registration to the navigator reference data set is difficult.
There may be numerous causes of such signal failures in the navigator volume. First, this may entail regions with large susceptibility jumps in the tissue in which the navigator signal contains anatomy-induced low MR signals. It may also be the case that the MR imaging sequence that is actually to be prospectively motion-corrected is recorded in a plurality of segments. These segments are separated by the navigator recording. In this case, residual magnetization may occur in the navigator volume as a result of the previous recording of a segment of the imaging sequence. This residual magnetization may greatly influence the image quality in the navigator volume during the recording of the navigator signals. This provides that the residual magnetization greatly influences the image quality of the navigator. If the signal voids in the navigator image data are too strong, robust detection of motion is no longer possible.